72 THE ANATOMY OF AN AUSTRALIAN LAND PLANARIAN. 
spenceri, but it is doubtful whether they are not principally a post-mortem effect, due 
to the action of re-agents upon a very delicate, gelatinous, or possibly fluid material. 
In sections of sponges very similar appearances are of common occurrence, and are 
undoubtedly due to shrinkage. In considering this question, von Graff's observations 
on the perivisceral fluid, quoted above, are of great moment. In the Rhabdocelida, at 
any rate, we know that in the living animal there is a fluid containing floating cells, 
and probably itself more or less albuminous. The effects of ordinary preservative 
re-agents upon such a fluid would be to cause it to contract and then harden in the 
form of a network of strands, in which the floatmg cells would be entangled. Then 
we have a complete explanation of the appearances often seen im sections. It appears 
to me that the published observations of Hallez, Jima and von Graff, pomt to the 
correctness of this conclusion. Certainly cells exist in the ground substance of the 
connective tissue. Their nuclei are plainly visible in considerable numbers in all 
preparations, and not infrequently the protoplasmic body of the cell, sometimes more 
or less stellate in form, 1s visible around the nucleus. The existence of connective 
tissue fibres is, it appears to me, entirely unproved, and the connective tissue probably 
consists of a more or less fluid, finely granular ground substance, with nucleated cells 
imbedded in it. Possibly, as in the case of the sponges, such for example as 
Stelospongos flabelliformis (10), the connective tissue cells may in some forms be 
connected by long processes, thus forming a network of cells, but the network usually 
seen in sections resembles rather a post-mortem effect. In the land Planarians the 
amount of connective tissue is very small, owing to the great development of the 
muscular fibres. 
F.—The Mucous Glands.—In the deep muscular layer, around the alimentary 
canal, there is an irregular zone of scattered glandular tissue (Figs. 4 and 15, s/.g.), 
which is at once recognised by its highly granular appearance and its deep staining 
with borax carmine. ‘This tissue is composed of irregularly scattered, large, nucleated 
cells of irregular outline, three of which are represented in figure 27. ‘The cells are 
eften seen, as shown in the figure, to be connected with threads or rods of mucous 
on its way to the surface. The mucous rods are long and slender, and can be traced, 
as shown in figure 15, through all the overlying tissues to the epidermis, where they 
are connected with the blocks of mucous lyimg between the epidermic cells as 
already described. Histologically the mucous rods partake of the characters of the 
unicellular glands which gave them origin, being very granular and staiming very 
deeply with borax carmine. 
Thus it appears that the slime with which the body of the land Planarians is so 
abundantly covered, and which is left behind like the trail of a snail wherever the 
animal crawls, is the secretion of a number of unicellular glands lying deep down 
below the epidermis. 
